1. Field of the Invention
The invention relates to an electric power steering system that includes a ball screw device.
2. Description of the Related Art
There is a rack-assist-type electric power steering system (EPS) that converts the rotation of a motor shaft, through which a rack shaft passes and which is rotated by a motor, into axial movement of the rack shaft using a ball screw device to supply assist force to a steering system.
In the EPS described above, the ball screw device is formed by arranging a plurality of balls between a ball screw nut secured to the motor shaft and a threaded portion formed in the rack shaft. When a threaded portion formed in the inner periphery of the ball screw nut and the threaded portion formed in the outer periphery of the rack shaft face each other, a helical ball-rolling path is formed. Each ball arranged in the ball-rolling path rolls under load. Thus, the rotation of the ball screw nut is converted into axial movement of the rack shaft.
For example, Japanese Patent Application Publication No. 2006-256414 (JP-A-2006-256414) describes a structure for securing a ball screw nut to a motor shaft. According to JP-A-2006-256414, the ball screw nut is fitted into a housing recess, formed in the inner periphery of the motor shaft, in the axial direction, and the axial position of the ball screw nut is determined by a locknut. Employment of this structure makes it possible to reduce the axial length.
However, employment of the above-described structure causes an increase in the diameter of a portion of the motor shaft, at which the ball screw nut and the motor shaft overlap each other. Further, clamping the ball screw nut using the locknut may cause deformation of the ball screw nut, resulting in distortion of a thread groove that forms a ball-rolling path. The distortion hinders smooth rolling of each ball in the ball-rolling path, which may cause abnormal sound, vibration, and deterioration of steering feel.
Therefore, for example, Japanese Patent Application Publication No. 6-255501 (JP-A-6-255501) describes a structure in which flanges that extend radially outward are formed at axial ends of a ball screw nut and a motor shaft, and the flanges are fastened to each other to connect the ball screw nut and the motor shaft to each other. This structure makes it possible to secure the ball screw nut and the motor shaft to each other in such a manner that the ball screw nut and the motor shaft are not allowed to rotate relative to each other, without causing deformation of the ball screw nut.
However, in the above-described structure, the flanges need to be fastened to each other with a plurality of bolts. Fastening the flanges to each other with a plurality of bolts is cumbersome. Therefore, it is difficult to increase the productivity.
Japanese Patent Application Publication No. 2007-239782 (JP-A-2007-239782) describes a structure in which a threaded portion is formed in the outer periphery of a ball screw nut and the ball screw nut is screwed to the inner periphery of a motor shaft. Employment of this structure makes it possible to disperse stress applied to the ball screw nut to the entirety of the outer periphery of the ball screw nut. Thus, it is possible to restrict deformation of the ball screw nut and distortion of a ball-rolling path due to the deformation. In addition, it is possible to improve the work efficiency during production.
However, it is difficult to form a threaded portion in the entirety of the outer periphery of a ball screw nut that has high strength and rigidity. In addition, providing the ball screw nut in a motor shaft causes upsizing of a portion of the motor shaft at which the ball screw nut is located. Further, the motor rotational direction is frequently changed in an EPS. Therefore, the frequent change in the motor rotational direction may generate a force that loosens the ball screw nut that is screwed to the motor shaft. However, countermeasures such as using a locknut as a rotation prevention device as described in JP-A-2007-239782 and using a setscrew that radially passes through the motor shaft and the ball screw nut may cause distortion of a ball-rolling path of the ball screw nut. On this point, there is still room for improvement.